Such a device, as described in my earlier U.S. Pat. Nos. 3,499,676 and 4,173,367 as well as in German Pat. Nos. 2,708,786, 2,708,787, and 2,708,788, has a shackle on which a pivot member is pivotal about a pickup axis. This pickup member has a generally radially extending actuation arm and an angularly elongated and outwardly open pickup slot lying generally in a plane perpendicular to the axis and having a relatively wide end permitting radial passage of the head of the bolt on the body into and out of the slot and an angularly oppositely relatively narrow end internally sufficiently wide to accommodate the head but externally insufficiently wide to permit radial passage of the head out of the slot. The slot is internally sufficiently wide between the ends to permit the head to pass angularly along the slot between the ends with the shaft extending out of the slot. Thus the actuation member can be fitted over the head of the bolt extending from the body by fitting the bolt into the wide end of the slot, then the member can be pivoted about the pivot axis to capture the head of the bolt in the slot, whereupon the concrete block or the like can be lifted by a crane connected to the shackle. Such an arrangement is relatively useful in the concentration of large prefabricated structures, as it allows the precast concrete body to be picked up in a very simple yet extremely safe manner. The pickup device can relatively easily be connected to and disconnected from the bolt, but has safety features which make it impossible for it to become disconnected from the body when under load.
The shackle of such an element normally is relatively long, having a pair of long sides that extend perpendicular to the pivot axis, and a pair of short sides bridging these long sides. One of the short sides forms the pivot axis and the other short side is the element over which the cable or grab of the lifting device is engaged. The shackle must be long enough to allow the actuation arm to pivot through it between the blocking and unblocking positions.
It is also known to provide such a device with a locking bolt slidable in a normally secantally extending guide in the member. This bolt can move between an inner position blocking the wide end of the slot and an outer freeing position clear of this wide end of the slot. When in the blocking position it makes it impossible for a bolt engaged in the slot to move from it, thereby making dropping of an object carried by the pickup device impossible. In such an arrangement the slot of the pickup member normally only extends over 90.degree. relative to the axis. Thus the pickup member can only pivot through 90.degree. between the position where it is dropped down on the pickup bolt and the position where it is locked to the pickup bolt with same in the narrow end of the slot.
The problem with such devices is that it is still possible occasionally for a picked-up body to be released by the pickup member according to this invention. This normally only occurs during accidental freak situations, or from misuse of the device; nonetheless it can happen. The most common accident is when the chain or hook that normally engages over the upper short side of the shackle becomes hooked over one of the long sides of the shackle or becomes wedged between the shackle and the pickup member so that the entire device is twisted seriously to the side when lifted. This can result in overstressing parts and ruining the pickup device.